Asymmetric tibial components for a knee prosthesis

ABSTRACT

An orthopaedic tibial prosthesis includes a tibial baseplate with an asymmetric periphery which promotes proper positioning and orientation on a resected tibia, while also facilitating enhanced kinematics, soft-tissue interaction, and long-term fixation of the complete knee prosthesis. The asymmetric baseplate periphery is sized and shaped to substantially match portions of the periphery of a typical resected proximal tibial surface, such that proper location and orientation is evident by resting the baseplate on the tibia. The baseplate periphery provides strategically positioned relief and/or clearance between the baseplate periphery and bone periphery, such as in the posterior-medial portion to prevent deep-flexion component impingement, and in the anterior-lateral portion to avoid undue interaction between the anatomic iliotibial band and prosthesis components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under Title 35, U.S.C. §119(e) ofU.S. Provisional Patent Application Ser. No. 61/381,800, filed on Sep.10, 2010 and entitled TIBIAL PROSTHESIS FACILITATING ROTATIONALALIGNMENT, and U.S. Provisional Patent Application Ser. No. 61/367,375,filed on Jul. 24, 2010 and entitled TIBIAL PROSTHESIS, the entiredisclosures of which are hereby expressly incorporated by referenceherein.

BACKGROUND

1. Technical Field

The present disclosure relates to orthopaedic prostheses and,specifically, to tibial components in a knee prosthesis.

2. Description of the Related Art

Orthopaedic prostheses are commonly utilized to repair and/or replacedamaged bone and tissue in the human body. For example, a kneeprosthesis may include a tibial baseplate that is affixed to a resectedor natural proximal tibia, a femoral component attached to a resected ornatural distal femur, and a tibial bearing component coupled with thetibial baseplate and disposed between the tibial baseplate and femoralcomponent. Knee prostheses frequently seek to provide articulationsimilar to a natural, anatomical articulation of a knee joint, includingproviding a wide range of flexion.

The tibial insert component, sometimes also referred to as a tibialbearing or meniscal component, is used to provide an appropriate levelof friction and contact area at the interface between the femoralcomponent and the tibial bearing component. For a knee prosthesis toprovide a sufficient range of flexion with a desirable kinematic motionprofile, the tibial bearing component and tibial baseplate must be sizedand oriented to interact appropriately with the femoral component of theknee prosthesis throughout the flexion range. Substantial design effortshave been focused on providing a range of prosthesis component sizes andshapes to accommodate the natural variability in bone sizes and shapesin patients with orthopaedic prostheses, while preserving flexion rangeand desired kinematic motion profile.

In addition to facilitating implantation and providing enhancedkinematics through manipulation of the size and/or geometry ofprosthesis components, protection and/or preservation of soft tissues inthe natural knee joint is also desirable.

A given prosthetic component design (i.e., a tibial baseplate, tibialbearing component, or femoral component) may be provided to a surgeon asa kit including a variety of different sizes, so that the surgeon maychoose an appropriate size intraoperatively and/or on the basis ofpre-surgery planning. An individual component may be selected from thekit based upon the surgeon's assessment of fit and kinematics, i.e., howclosely the component matches the natural contours of a patient's boneand how smoothly the assembled knee joint prosthesis functions inconjunction with adjacent soft tissues and other anatomical structures.Soft tissue considerations include proper ligament tension andminimization of soft tissue impingement upon prosthetic surfaces, forexample.

In addition to prosthetic sizing, the orientation of a prostheticcomponent on a resected or natural surface of a bone also impactssurgical outcomes. For example, the rotational orientation of a tibialbaseplate and tibial bearing component with respect to a resectedproximal tibia will affect the interaction between the correspondingfemoral prosthesis and the tibial bearing component. The nature andamount of the coverage of a tibial baseplate over specific areas of theresected proximal tibia will also affect the fixation of the implant tothe bone. Thus, substantial design efforts have been focused onproviding prosthetic components which are appropriately sized for avariety of patient bone sizes and are adapted to be implanted in aparticular, proper orientation to achieve desired prosthesis performancecharacteristics.

SUMMARY

The present disclosure provides an orthopaedic tibial prosthesisincluding a tibial baseplate with an asymmetric periphery which promotesproper positioning and orientation on a resected tibia, while alsofacilitating enhanced kinematics, soft-tissue interaction, and long-termfixation of the complete knee prosthesis. The asymmetric baseplateperiphery is sized and shaped to substantially match portions of theperiphery of a typical resected proximal tibial surface, such thatproper location and orientation is evident by resting the baseplate onthe tibia. The baseplate periphery provides strategically positionedrelief and/or clearance between the baseplate periphery and boneperiphery, such as in the posterior-medial portion to preventdeep-flexion component impingement, and in the anterior-lateral portionto avoid undue interaction between the anatomic iliotibial band andprosthesis components.

In one form thereof, the present invention provides a tibial prosthesiscomprising: a distal surface; a proximal surface generally opposite thedistal surface, the proximal surface having a lateral compartment and amedial compartment; and a peripheral wall extending between the distaland the proximal surface, the peripheral wall defining an anterior edge;a lateral posterior edge generally opposite the anterior edge andforming a posterior boundary of the lateral compartment; a medialposterior edge generally opposite the anterior edge and forming aposterior boundary of the medial compartment; a lateral peripheryextending from the anterior edge to the lateral posterior edge, thelateral periphery defining a plurality of adjacent lateral arcs, anadjacent pair of the plurality of adjacent lateral arcs defining a firstlateral radius and a second lateral radius, respectively, the firstlateral radius larger than the second lateral radius by at least 100%,whereby the lateral periphery is relatively boxy; and a medial peripheryextending from the anterior edge to the medial posterior edge, themedial periphery defining a plurality of adjacent medial arcs, anadjacent pair of the plurality of adjacent medial arcs defining a firstmedial radius and a second medial radius, respectively, the first medialradius larger than the second medial radius by less than 75%, wherebythe medial periphery is generally rounded.

In another form thereof, the present invention provides a tibialprosthesis comprising: a distal surface; a proximal surface generallyopposite the distal surface; and a peripheral wall extending between thedistal and the proximal surface, the peripheral wall defining ananterior edge; a lateral periphery including: a lateral edge defining asubstantially perpendicular tangent with respect to the anterior edge,an anterior-lateral corner traversing an angular sweep between theanterior edge and the lateral edge to define a boxy corner peripheryhaving an anterior-lateral corner edge length, and a posterior-lateralcorner extending away from the lateral edge and the anterior-lateralcorner; and a medial periphery including: a medial edge defining asubstantially perpendicular tangent with respect to the anterior edge,an anterior-medial corner traversing an angular sweep between theanterior edge and the medial edge to define a rounded corner peripheryhaving an anterior-medial corner edge length that is longer than theanterior-lateral corner edge length, in which the angular sweep betweenthe anterior edge and the medial edge is similar to the angular sweepbetween the anterior edge and the lateral edge, and a posterior-medialcorner extending away from the medial edge and the anterior-medialcorner.

In yet another form thereof, the present invention provides a tibialprosthesis comprising an asymmetric prosthesis periphery, the peripherycomprising: an anteroposterior axis dividing the prosthesis peripheryinto a medial compartment and a lateral compartment; an anterior edgedisposed between the medial compartment and the lateral compartment; alateral posterior edge generally opposite the anterior edge and forminga posterior boundary of the lateral compartment; a medial posterior edgegenerally opposite the anterior edge and forming a posterior boundary ofthe medial compartment; a lateral periphery extending from the anterioredge to the lateral posterior edge, the lateral periphery defining: ananterior-lateral arc having an anterior-lateral arc center; and alateral arc having a lateral arc center, the lateral arc defining atangent parallel to the anteroposterior axis; a medial peripheryextending from the anterior edge to the medial posterior edge, themedial periphery defining an anterior-medial arc having ananterior-medial arc center; and a medial arc having a medial arc center,the medial arc defining a tangent parallel to the anteroposterior axis,a mediolateral axis defining the longest line segment within theprosthesis periphery that is also perpendicular to the anteroposterioraxis, the anterior-lateral arc center disposed between the mediolateralaxis and the anterior edge, the anterior-medial arc center disposedposterior of the mediolateral axis.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1A is an exploded, perspective view of a tibial baseplate andtibial bearing component in accordance with the present disclosure;

FIG. 1B is an assembled, perspective view of the tibial baseplate andtibial bearing component shown in FIG. 1A;

FIG. 2A is a top plan view of the peripheries of a set of nine tibialbaseplates made in accordance with the present disclosure, in which theperipheries are shown to scale according to the illustrated scales inmillimeters in the bottom and right-hand margins of the page;

FIG. 2B is a top plan view of the periphery of a tibial baseplate madein accordance with the present disclosure;

FIG. 2C is a graph illustrating the asymmetric growth of theposterior-medial compartment for the tibial baseplates shown in FIG. 2A;

FIG. 2D is a graph illustrating the asymmetric growth of theposterior-lateral compartment for the tibial baseplates shown in FIG.2A;

FIG. 3A is top plan view of a periphery of a tibial baseplate made inaccordance with the present disclosure, illustrating various arcsdefined by the periphery;

FIG. 3B is a partial, top plan view of the periphery shown in FIG. 3A,illustrating an alternative lateral corner periphery;

FIG. 3C is a partial, top plan view of the periphery shown in FIG. 3A,illustrating an alternative medial corner periphery;

FIG. 3D is a top plan view of the periphery of a tibial baseplate madein accordance with the present disclosure, illustrating medial andlateral surface area calculations without a PCL cutout;

FIG. 4A is a top plan view of a tibial baseplate made in accordance withthe present disclosure;

FIG. 4B is a side elevation view of the tibial baseplate shown in FIG.4A;

FIG. 5 is a top plan view of a resected proximal tibial surface with aprosthetic tibial baseplate component and tibial bearing component madein accordance with the present disclosure mounted thereon;

FIG. 6 is a top plan view of a resected proximal tibial surface with aproperly sized tibial trial component thereon;

FIG. 7 is a side, elevation view of the tibia and trial component shownin FIG. 6; and

FIG. 8 is a side, elevation view of the tibial components shown in FIG.1A, in conjunction with a femoral component.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate exemplary embodiments of the invention, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

The present disclosure provides an asymmetric knee joint prosthesiswhich facilitates proper rotational and spatial orientation of a tibialbaseplate and tibial bearing component upon a resected proximal tibia,while also offering large-area contact with the resected proximal tibia.The prosthesis permits a wide range of flexion motion, protects naturalsoft tissue proximate the knee joint prosthesis, and optimizes long termfixation characteristics of the prosthesis.

In order to prepare the tibia and femur for receipt of a knee jointprosthesis of the present disclosure, any suitable methods orapparatuses may be used. As used herein, “proximal” refers to adirection generally toward the torso of a patient, and “distal” refersto the opposite direction of proximal, i.e., away from the torso of thepatient.

As used herein, the “periphery” of a tibial prosthesis refers to anyperiphery as viewed in a top plan view, e.g., in a generally transverseanatomical plane. Alternatively, the periphery of a tibial prosthesismay be any periphery as viewed in bottom plan view, e.g., in a generallytransverse plane and looking at the distal surface adapted to contact aresected proximal surface of a tibial bone.

As used herein, the term “centroid” or “geometric center” refers to theintersection of all straight lines that divide a given area into twoparts of equal moment about each respective line. Stated another way, ageometric center may be said to be the “average” (i.e., arithmetic mean)of all points of the given area. Stated yet another way, the geometriccenter is a point in a two dimensional figure from which the sum of thedisplacement vectors of all points on the figure equals zero.

As used herein, a “disparity” or “difference” between two numericalvalues (e.g., one value “larger” or “smaller” than another), typicallyexpressed as a percentage, is the difference between the two valuesdivided by the smaller of the two values. For example, a smallerquantity having value 75 and a larger quantity having value 150 wouldhave a percentage disparity of (150-75)/75, or 100%.

Referring to FIG. 5, tibia T includes tibial tubercle B havingmediolateral width W, with tubercle midpoint P_(T) located on tubercle Bapproximately halfway across width W. While tubercle B is shown ashaving midpoint P_(T) at the “peak” or point of maximum anterioreminence, it is recognized that midpoint P_(T) of tibia T may be spacedfrom such a peak. Tibia T also includes attachment point C_(P)representing the geometric center of the attachment area between theanatomic posterior cruciate ligament (PCL) and tibia T. Recognizing thatthe PCL typically attaches to a tibia in two ligament “bundles,” one ofwhich is relatively anterior, lateral and proximal and the other ofwhich relatively posterior, medial and distal, attachment point C_(P) iscontemplated as representing the anterior/lateral attachment area in anexemplary embodiment. However, it is contemplated that theposterior/medial attachment area, or the entire attachment area, couldbe used.

As used herein, “anterior” refers to a direction generally toward thefront of a patient. “Posterior” refers to the opposite direction ofanterior, i.e., toward the back of the patient.

In the context of patient anatomy, “home axis” A_(H) refers to agenerally anteroposterior axis extending from posterior point C_(P) toan anterior point C_(A), in which anterior point C_(A) is disposed ontubercle B and medially spaced from tubercle midpoint P_(T) by an amountequal to W/6. Stated another way, anterior point C_(A) is laterallyspaced by an amount equal to W/3 from the medial end of mediolateralwidth W, such that point C_(A) lies on the “medial third” of theanterior tibial tubercle.

In the context of a prosthesis, such as tibial baseplate 12 describedbelow, “home axis” A_(H) refers to an axis oriented with respect tobaseplate 12 such that the baseplate home axis A_(H) of baseplate 12 isaligned with home axis A_(H) of tibia T after implantation of baseplate12 in a proper rotational and spatial orientation (as shown in FIG. 5).In the illustrative embodiments shown in FIG. 3 and described in detailbelow, home axis A_(H) bisects PCL cutout 28 at the posterior edge ofperiphery 200 of tibial plateau 18 (FIG. 5), and bisects anterior edge202 at the anterior edge of periphery 200 of tibial plateau 18. It iscontemplated that home axis A_(H) may be oriented to other baseplatefeatures, it being understood home axis A_(H) of baseplate 12 ispositioned such that that proper alignment and orientation of baseplate12 upon tibia T positions the home axis A_(H) of baseplate 12 coincidentwith home axis A_(H) of tibia T.

Home axis A_(H) of tibial baseplate 12 may be said to be ananteroposterior axis, as home axis A_(H) extends generally anteriorlyand posteriorly when baseplate 12 is implanted upon tibia T. Tibialbaseplate also defines mediolateral axis A_(ML), which lies along thelongest line segment contained within periphery 200 that is alsoperpendicular to home axis A_(H) of baseplate 12. As described below,home axis A_(H) and mediolateral axis A_(ML) cooperate to define acoordinate system useful for quantifying certain baseplate features inaccordance with the present disclosure.

The embodiments shown and described with regard to FIGS. 1A, 1B, 3A, 4A,4B, 5 and 6 illustrate a left knee and associated features of aright-knee prosthesis, while the embodiments shown and described inFIGS. 2A, 2B and 3D illustrate the periphery of a right knee prosthesis.Right and left knee configurations are mirror images of one anotherabout a sagittal plane. Thus, it will be appreciated that all aspects ofthe prosthesis described herein are equally applicable to a left- orright-knee configuration.

1. Asymmetry of the Tibial Prosthesis.

Referring now to FIGS. 1A and 1B, tibial prosthesis 10 includes tibialbaseplate 12 and tibial bearing component 14. Tibial baseplate 12 mayinclude a stem or keel 16 (FIG. 4B) extending distally from proximaltibial plateau 18, or may utilize other fixation structures for securingbaseplate 12 to tibia T, such as distally extending pegs. Portions ofthe outer periphery defined by tibial plateau 18 closely correspond insize and shape with a resected proximal surface of tibia T, as describedin detail below.

Tibial bearing component 14 and tibial baseplate 12 have a particularasymmetry, with respect to home axis A_(H) (shown in FIG. 2A anddescribed above), that is designed to maximize tibial coverage for alarge proportion of knee-replacement candidates. This high level ofcoverage allows a surgeon to cover the largest possible area on theproximal resected surface of the tibia, which in turn offers maximumcoverage of cortical bone. Advantageously, the maximized coverage ofcortical bone facilitates superior support of tibial baseplate 12. Afirm, enduring fixation of tibial baseplate 12 to tibia T is facilitatedby large-area contact between the cortical and cancellous bone of tibiaT and distal surface 35 of tibial plateau 18 (FIG. 4B), which may becoated with porous ingrowth material and/or bone cement.

In an analysis of a several human specimens, variations in size andgeometry for a variety of anatomic tibial features were observed andcharacterized. Geometrical commonalities between anatomic features, orlack thereof, were noted. Mean tibial peripheral geometries werecalculated based on statistical analysis and extrapolation of thecollected anatomical data, in view of the observed geometricalcommonalities organized around anatomic home axis A_(H). Thesecalculated mean geometries were categorized by tibial size.

A comparison between the asymmetric peripheries for the present familyof prostheses and the calculated mean tibial geometries was conducted.Based on the results of this comparison, it has been found thatsubstantial tibial coverage can be achieved for a large proportion ofpatients using tibial components having asymmetric peripheries inaccordance with the present disclosure. Moreover, this coverage can beachieved with a relatively small number of sizes, even where particularportions of the prosthesis periphery is intentionally “pulled back” fromthe tibial periphery in order to confer other orthopaedic benefits.Further, the particular asymmetry of tibial baseplate 12 can be expectedto offer such coverage without overhanging any portion of the resectedsurface.

Thus, periphery 200 including the particular asymmetric profile asdescribed below confers the benefits of maximum coverage, facilitationof proper rotation (discussed below), and long-term fixation asdescribed herein. Such asymmetry may be demonstrated in various ways,including: by a comparison of adjacent radii in the medial and lateralcompartments of the asymmetric periphery; by a comparison of the edgelength in anterior-medial and anterior lateral corners of the periphery,for a comparable lateral and medial angular sweep; and by a comparisonof the location of radius centers for the anterior-medial andanterior-lateral corners with respect to a mediolateral axis. Variouscomparisons and quantifications are presented in detail below. Specificdata and other geometric details of the peripheries for the variousprosthesis sizes, from which the below-identified comparisons andquantifications are derived, may be obtained from the draw-to-scaleperipheries shown in FIG. 2A.

Advantageously, the asymmetry of tibial component 12 encourages properrotational orientation of baseplate 12 upon implantation thereof ontotibia T. As described in detail below, the asymmetry of periphery 200(FIG. 2A) of tibial plateau 18 is designed to provide a close match inselected areas of the lateral and medial compartments as compared to theanatomic bone. As such, a surgeon can select the largest possiblecomponent from among a family of different component sizes, such thatthe component substantially covers the resected tibia T with minimalgaps between the tibial periphery and component periphery 200, as wellas little or no overhang over any portions of the tibial periphery.Because the high congruence between prosthesis periphery 200 and thetibial periphery produces only a minimal gap between the peripheries (asshown in FIG. 5), tibial baseplate 12 cannot be rotated significantlywithout causing tibial plateau 18 to overhang beyond the periphery ofthe resected tibial surface. Thus, proper rotation of baseplate 12 canbe ascertained by the visual acuity between prosthesis periphery 200 andthe resected tibial surface.

The following examples and data are presented with respect to tibialbaseplate 12. However, as described in more detail below, tibial bearingcomponent 14 defines perimeter wall 54 which follows peripheral wall 25of baseplate 12 except where noted. Thus, it is appreciated that theconclusions, trends and design features gleaned from data relating tothe asymmetric periphery of tibial baseplate 12 also applies to theasymmetric periphery of tibial bearing component 14, except where statedotherwise.

Lateral compartment 20 and medial compartment 22 of tibial plateau 18are dissimilar in size and shape, giving rise to the asymmetry thereof.This asymmetry is designed so that peripheral wall 25 traces theperimeter of the resected proximal surface of tibia T, such that tibialplateau 18 covers a large proportion of the resected proximal tibialsurface as shown in FIG. 5. To achieve this large tibial coverage,tibial plateau 18 closely matches the periphery of tibia T in most areasas noted above. Nevertheless, as shown in FIG. 5, for example, a smallgap between periphery 200 of tibial plateau 18 and tibia T is formed toallow some freedom of positioning and rotational orientation. The gap isdesigned to have a substantially continuous width in most areas,including the anterior edge, anterior-medial corner, medial edge,lateral edge and lateral-posterior corner (all described in detailbelow).

However, certain aspects of the asymmetric shape are designed tointentionally deviate from the calculated anatomical shape to conferparticular features and advantages in the context of a complete,implanted knee prosthesis. Referring to FIG. 5, for example, tibialbaseplate 12 and tibial bearing component 14 have anterior-lateral“corners” (described in detail below) which are “pulled back” to creategap 56 between tibia T and prosthesis 10 in the anterior-lateral area ofthe resected surface of tibia T. Advantageously, gap 56 creates extraspace for “soft-tissue friendly” edges of prosthesis 10, therebyminimizing impingement of the iliotibial band. In an exemplaryembodiment, gap 56 may range from 0.5 mm for a small-size prosthesis(such as size 1/A described below), to 1 mm for a medium-sizedprosthesis (such as size 5/E described below), to 2 mm for a large-sizedprosthesis (such as size 9/J described below).

Similarly, the posterior edge of the medial compartment may be “pulledback” from the adjacent edge of tibia T to define gap 58. Gap 58 allowsextra space for adjacent soft tissues, particularly in deep flexion asdescribed below. Gap 58 also allows prosthesis 10 to be rotated about alateral pivot by a small amount, thereby offering a surgeon the freedomto displace medial compartment 22 posteriorly as required or desired fora particular patient. In an exemplary embodiment, gap 58 is about 4 mm.

As described in detail below, the asymmetrical periphery also provides alarge overall area for proximal surface 34 of baseplate 12, whichcreates sufficient space for large contact areas between tibial bearingcomponent 14 and femoral component 60 (FIG. 8).

a. Medial/Lateral Peripheral Curvatures

The particular asymmetric shape of tibial plateau 18 (and of tibialbearing component 14, which defines a similar periphery as describedbelow) gives rise to a generally “boxy” or angular periphery in lateralcompartment 20, and a “rounded” or soft periphery in medial compartment22.

Turning to FIG. 3A, the periphery 200 of tibial plateau 18 surroundslateral compartment 20 and medial compartment 22, each of which define aplurality of lateral and medial arcs extending between anterior edge 202and lateral and medial posterior edges 204, 206 respectively. In theillustrative embodiment of FIG. 3A, anterior edge 202, lateral posterioredge 204 and medial posterior edge 206 are substantially planar andparallel for ease of reference. However, it is contemplated that edges202, 204, 206 may take on other shapes and configurations within thescope of the present disclosure, such as angled or arcuate.

In the exemplary embodiment of FIG. 3A, lateral compartment 20 includesfive separate arcs including lateral anterior edge arc 208,anterior-lateral corner arc 210, lateral edge arc 212, posterior-lateralcorner arc 214, and lateral posterior edge arc 216. Each of lateral arcs208, 210, 212, 214 and 216 defines angular sweep 1L, 2L, 3L, 4L and 5L,respectively, having radii R1L, R2L, R3L, R4L and R5L respectively. Aradius of a particular angular sweep extends from the respective radiuscenter (i.e., one of centers C1L, C2L, C3L, C4L and C5L) to periphery200. Radii R1L, R2L, R3L, R4L and R5L each remain unchanged throughoutthe extent of angular sweeps 1L, 2L, 3L, 4L and 5L, respectively.

Similarly, medial compartment 22 includes three separate arcs includinganterior-medial corner arc 220, medial edge arc 222 andposterior-lateral corner arc 224, defining angular sweeps 1R, 2R and 3R,respectively having radii R1R, R2R and R3R respectively.

In FIG. 2A, peripheries 200 _(X) are shown for each of nineprogressively larger component sizes, with 200 ₁ being the periphery ofthe smallest size (size “1” or “A”) and 200 ₉ being the periphery of thelargest size (size “9” or “J”). For purposes of the present disclosure,several quantities and features of tibial baseplate 12 may be describedwith the subscript “X” appearing after the reference numeralcorresponding to a component size as set for in the Tables, Figures anddescription below. The subscript “X” indicates that the referencenumeral applies to all nine differently-sized embodiments described andshown herein.

In exemplary embodiments, medial and lateral radii may be any valuewithin the following ranges: for medial radius R1R_(X), between about 27mm and about 47 mm; for medial radius R2R_(X), between about 21 mm andabout 49 mm; for medial radius R3R_(X), between about 14 mm and about 31mm; for lateral radius R1L_(X), between about 46 mm and about 59 mm; forlateral radius R2L_(X), between about 13 mm and about 27 mm; for lateralradius R3L_(X) between about 27 mm and about 46 mm; for lateral radiusR4L_(X) between about 6 mm and about 14 mm; and for lateral radiusR5L_(X) between about 22 mm and about 35 mm.

In exemplary embodiments, medial and lateral angular extents or sweepsmay be any value within the following ranges: for medial angle 1R_(X),between about 13 degrees and about 71 degrees; for medial angle 2R_(X),between about 23 degrees and about 67 degrees; for medial angle 3R_(X),between about 23 degrees and about 90 degrees; for lateral angle 1L_(X),between about 11 degrees and about 32 degrees; for lateral angle 2L_(X),between about 42 degrees and about 63 degrees; for lateral angle 3L_(X),between about 23 degrees and about 47 degrees; for lateral angle 4L_(X),between about 36 degrees and about 46 degrees; and for lateral angle5L_(X), between about 28 degrees and about 67 degrees;

The unique asymmetry of periphery 200 defined by tibial plateau 18 canbe quantified in multiple ways with respect to the curvatures of lateraland medial compartments 20 and 22 as defined by the arrangement andgeometry of lateral arcs 208, 210, 212, 214, 216 and medial arcs 220,222, 224.

One measure of the asymmetry of periphery 200 is found in a simplecomparison of radii R2L and R1R, which are the anterior “corner” radiiof lateral and medial compartments 20 and 22 respectively. Generallyspeaking, a corner of a baseplate periphery may be said to be thatportion of the periphery where a transition from an anterior orposterior edge to a lateral or medial edge occurs. For example, in theillustrative embodiment of FIG. 3A, the anterior-lateral corner isprincipally occupied by anterior-lateral corner arc 210, which defines asubstantially medial-lateral tangent at the anterior end of arc 210 anda substantially anteroposterior tangent at the lateral end of arc 210.Similarly, the medial corner of periphery 200 is principally occupied byanterior-medial corner arc 220, which defines a substantiallymedial-lateral tangent at the anterior end of arc 220 and a moreanteroposterior tangent at the lateral end of arc 220. For somepurposes, the anterior-medial corner of periphery 200 may be said toinclude a portion of medial edge arc 222, as described below.

A periphery corner may also be defined by a particular angular sweepwith respect to an anteroposterior reference axis. Such reference axismay extend posteriorly from an anterior-most point of a tibialprosthesis (e.g., from the center of anterior edge 202 of periphery 200)to divide the prosthesis into medial and lateral halves. In asymmetrical prosthesis, the anteroposterior reference axis is the axisof symmetry.

In the illustrative embodiment of FIG. 3A, the anteroposterior referenceaxis may be home axis A_(H), such that the anterior-medial corner ofperiphery 200 occupies some or all of the 90-degree clockwise angularsweep between home axis A_(H) (at zero degrees, i.e., the beginning ofthe clockwise sweep) and mediolateral axis A_(ML) (at 90 degrees, i.e.,the end of the sweep). Similarly, the anterior-lateral corner ofperiphery 200 occupies some or all of the 90-degree counter-clockwiseangular sweep between home axis A_(H) and mediolateral axis A_(ML).

For example, the anterior-medial and anterior-lateral corners may eachoccupy the central 45 degree angular sweep of their respective 90-degreeangular sweeps as described above. Thus, the anterior-lateral corner ofperiphery 200 would begin at a position rotated 22.5 degreescounter-clockwise from home axis A_(H) as described above, and would endat 67.5 degrees counter-clockwise from home axis A_(H). Similarly, theanterior-medial corner would begin at a 22.5-degree clockwise rotationand end at a 67.5 degree clockwise rotation.

It is contemplated that the anterior-lateral and anterior-medial cornersmay occupy any angular sweep as required or desired for a particulardesign. For purposes of comparison between two corners in a givenprosthesis periphery, however, a comparable angular sweep for thelateral and medial sides is envisioned, i.e., the extent and location ofthe compared angles may be “mirror images” of one another about ananteroposterior axis. For example, in a comparison of anterior-lateraland anterior-medial radii R2L, R1R, it is contemplated that suchcomparison is calculated across lateral and medial angular sweeps whicheach begin and end at similar angular end points with respect to thechosen reference axis (e.g., home axis A_(H)).

As best seen in FIGS. 3A and 5, one aspect of the asymmetric peripheryof baseplate 12 arises from R1R_(X) being substantially larger thanR2L_(X). Table 1, below, also includes a comparison of radii R1R_(X) andR2L_(X) across nine exemplary component sizes, demonstrating thatdifference Δ−12RL between radius R1R_(X) and radius R2L_(X) may be aslittle as 48%, 76% or 78%, and may be as much as 102%, 103% or 149%. Itis contemplated that radius R1R_(X) may be larger than radius R2L_(X) byany percentage value within any range defined by the listed values.

TABLE 1 Comparisons of Values of Respective Medial and Lateral AnteriorCorner Radii Δ-12RL SIZE R1R vs. R2L 1/A 103.0% 2/B 149.2% 3/C  82.4%4/D  74.6% 5/E  90.9% 6/F  78.6% 7/G 102.2% 8/H  86.5% 9/J  48.1% AVG 90.6% All Δ values are expressed as the difference between a given pairof radii, expressed as a percentage of the smaller of the two radii

Stated another way, the smaller R2L_(X) makes a sharper turn, therebyimparting a relatively more “boxy” appearance to the anterior corner oflateral compartment 20, while the relatively larger radius R1R_(X) makesa more gradual turn that imparts a more “rounded” appearance to theanterior corner of medial compartment 22. In the exemplary nine sizesillustrated in FIG. 2A and shown in Table 1, an average disparitybetween the lateral and medial anterior corner radii R2L_(X) and R1R_(X)is greater than 90%. In some sizes of periphery 200 _(X), theanterior-medial “corner” making the more gradual turn may also includesmedial edge arc 222.

As described in detail below, this “rounded-medial/boxy-lateral”asymmetry of the anterior corners of tibial plateau facilitates andencourages proper rotational orientation and positioning of baseplate 12upon tibia T upon implantation by allowing periphery 200 to closelymatch the periphery of a typical resected tibia T (FIG. 5), while alsomaximizing the surface area of proximal surface 34 of tibial plateau toallow for use of a tibial bearing component 14 with a concomitantlylarge proximal surface area.

As noted above, the small-radius “corner” defined by angle 2L may beconsidered to have a similar angular sweep as a large-radius “corner”defined by angles 1R, 2R (or a combination of portions thereof) forpurposes of comparing the two radii. Given this comparable angularsweep, another measure of the asymmetry defined by the medial andlateral anterior corners is the arc length of the corners. Moreparticularly, because medial radii R1R_(X) and R2R_(X) are larger thanlateral radius R2L_(X) (as described above), it follows that the medialcorner has a larger arc length as compared to the lateral corner arclength for a given angular sweep.

Moreover, while the peripheries of lateral and medial compartments 20,22 are shown as being generally rounded and therefore definingrespective radii, it is contemplated that an asymmetric periphery inaccordance with the present disclosure need not define a radius per se,but rather could include one or more straight line segments which, onthe whole, define asymmetric corner edge lengths in the medial andlateral compartments. Referring to FIG. 3B, for example, it iscontemplated that an alternative anterior lateral corner 210′ could becomprised of three line segments 210A, 210B, 210C which cooperate tospan angular extent 2L. Similarly, an alternative anterior medial corner220′ could be comprised of three line segments 220A, 220B, 220C whichcooperate to span angular extent 1R. Any of the other arcs which defineperiphery 200 could be similarly configured as one or more linesegments. In the variant illustrated by FIGS. 3B and 3C, the differencebetween corner radii would not be an appropriate measure of asymmetrybecause the straight line segments would not define radii. Asymmetry ofthe medial and lateral anterior corners would instead be quantified bycomparison of the respective lengths of the medial and lateral corneredges across comparable medial and lateral angular extents.

Yet another way to quantify the asymmetry of the anterior corner arcs(i.e., anterior-lateral corner arc 210 and anterior-medial corner arc220) is to compare the distance of the lateral and medial radius centersC2L and C1R respectively, from anterior edge 202 and/or mediolateralaxis A_(ML) (FIG. 3A). In the boxy anterior-lateral corner, centerC2L_(X) of radius R2L_(X) is anterior of mediolateral axis A_(ML) andrelatively close to anterior edge 202. For the rounded, anterior-medialcorner, centers C1R_(X) and C2R_(X) of radii R1R_(X) and R2R_(X),respectively, are posterior of mediolateral axis A_(ML) and relativelyfar from anterior edge 202.

Another metric for quantifying the “boxy vs. rounded” asymmetry ofperiphery 200 is a comparison between ratios of adjacent radii. In themore boxy lateral compartment 20, pairs of adjacent radii define largeratios because the large edge radii (i.e., of lateral anterior edge arc208, lateral edge arc 212 and lateral posterior edge arc 216) are muchlarger than the adjacent corner radii (i.e., of anterior-lateral cornerarc 210 and posterior-lateral corner arc 214). On the other hand, in themore rounded medial compartment 22, pairs of adjacent radii define smallratios (i.e., nearly 1:1) because the radii of the medial arcs (i.e.,anterior-medial corner arc 220, medial edge arc 222 and posterior-medialcorner arc 224) are of similar magnitude.

In the illustrated embodiment of FIG. 3A, lateral edge arc 212 isconsidered an “edge” because arc 212 defines tangent 212A which issubstantially perpendicular to anterior edge 202. Just as a “corner” maybe considered to be the portion of periphery 200 which makes atransition from anterior or posterior to medial or lateral, an edge isthat portion of periphery 200 which encompasses the anterior, posterior,medial or lateral terminus of periphery 200.

Similarly, medial edge arc 222 defines tangent 222A which is alsosubstantially perpendicular to anterior edge 202. The medial “edge” ofperiphery 200 may be part of the same arc that extends around theanterior-medial corner and/or the anterior-lateral corner, as the medialarcs are similar. Indeed, as noted herein, medial compartment 22 mayhave a single arc which extends from anterior edge 202 to medialposterior edge 206.

Table 2 shows a comparison between adjacent-radii ratios for lateral andmedial compartments 20 and 22. For each adjacent pair of radii, thedifference between the radii magnitudes are expressed as a percentage ofthe smaller radius of the pair, as noted above.

TABLE 2 Comparisons of Values of Respective Pairs of BaseplatePeripheral Radii Δ-12R Δ-23R Δ-12L Δ-23L Δ-34L Δ-45L R1R vs. R2R vs. R1Lvs. R2L vs. R3L vs. R4L vs. SIZE R2R R3R R2L R3L R4L R5L 1/A 18.3% 58.6%337.3% 141.8% 323.5% 194.1% 2/B 49.0% 62.0% 254.1% 96.7% 361.5% 315.4%3/C 24.0% 48.8% 247.1% 58.8% 203.4% 214.6% 4/D 44.2% 34.4% 207.0% 59.2%213.9% 244.4% 5/E 23.3% 57.9% 151.5% 80.6% 250.0% 250.0% 6/F 46.5% 37.6%122.6% 42.9% 222.6% 260.2% 7/G 25.3% 38.9% 110.8% 64.5% 264.3% 176.2%8/H 73.6% 21.3% 109.0% 80.9% 198.1% 142.6% 9/J 21.9% 61.2% 70.4% 68.5%264.0% 172.0% AVG 36.2% 46.7% 178.9% 77.1% 255.7% 218.8% All Δ valuesare expressed as the difference between a given pair of radii, expressedas a percentage of the smaller of the two radii

As illustrated in Table 2, the “boxy” periphery of lateral compartment20 gives rise to disparity values Δ−12L, Δ−23L, Δ−34L and Δ−45L that areat least 42%, 48% or 59%, and as great as 323%, 337% or 362%. It iscontemplated that the disparity between a pair of adjacent radii in theboxy periphery of lateral compartment 20 may be any percentage valuewithin any range defined by any of the listed values. It is alsocontemplated that the lateral disparity values may be substantiallyhigher, as required or desired for a particular application.

Meanwhile, the “rounded” periphery of medial compartment 22 gives riseto disparity values Δ−12R and Δ−23R that are as small as 21%, 23% or25%, and no greater than 61%, 62% or 74%. It is contemplated that thedisparity between a pair of adjacent radii in the rounded periphery ofmedial compartment 22 may be any value within any range defined by anyof the listed values. It is also contemplated that the medial disparityvalues may be less than 21%, and as little as zero %, as required ordesired for a particular application.

Moreover, the boxy shape of lateral compartment 20 and rounded shape ofmedial compartment 22 is also demonstrated by the number of arcs used todefine the portion of periphery 200 in lateral and medial compartments20, 22. In lateral compartment 20, five arcs (i.e., arcs 208, 210, 212,204, 216) are used to define the lateral periphery, which is indicativeof anterior, lateral and posterior “sides” of a box joined by therelatively sharp transitions of corner arcs 210, 214. On the other hand,medial compartment 22 uses only three radii (i.e., 220, 222, 224),leaving no clear definition of any box “sides” or other transitions.Indeed, it is contemplated that medial compartment 22 could joinanterior edge 202 to medial posterior edge 206 by a single radius withinthe scope of the present disclosure.

b. Surface Area of Medial and Lateral Baseplate Compartments

Referring still to FIG. 3A, yet another characterization of theasymmetry of periphery 200 arises from disparities in surface area forlateral and medial compartments 20, 22. For purposes of the presentdisclosure, surface area of lateral compartment SAL is that areacontained within periphery 200, and on the lateral side of home axisA_(H). Similarly, the surface area of medial compartment 22 is that areacontained within periphery 200, and on the medial side of home axisA_(H).

In an exemplary embodiment, lateral surface area SAL_(X) may be aslittle as 844 mm² or may be as much as 1892 mm², or may be any areawithin the range defined by the foregoing values. In an exemplaryembodiment, medial surface area SAM_(X) may be as little as 899 mm² ormay be as much as 2140 mm², or may be any area within the range definedby the foregoing values.

Surfaces areas SAL and SAM do not include any of the area occupied byPCL cutout 28, as any such area is not within periphery 200. However,the asymmetry of surface areas SAL and SAM arises primarily from thedifferences in the geometry and placement of arcs 208, 210, 212, 214,216, 220, 222, 224 rather than from any asymmetry of PCL cutout 28. Inthe illustrative embodiments of FIG. 2A, for example, PCL cutout 28 _(X)is symmetrical with respect to home axis A_(H), but extends furtherposteriorly in medial compartment 22.

Thus, it is contemplated that the asymmetry of surfaces areas SAL, SAMare little changed by exclusion of the PCL cutout 28 from the areacalculation. As illustrated in FIG. 3D, PCL cutout 28 is effectivelyexcluded from calculation by extrapolating the line formed by lateralposterior edge 204 and medial posterior edge 206 inwardly to intersectwith home axis A_(H). In lateral compartment 20, such extrapolationcooperates with the lateral side of PCL cutout 28 to define lateral fillarea 80. In medial compartment 22, such extrapolation cooperates withthe medial side of PCL cutout 28 to define medial fill area 82.

In the illustrative embodiment of FIG. 3D, lateral surface area SAL_(X)″may be as little as 892 mm² or may be as much as 2066 mm², or may be anyarea within the range defined by the foregoing values. In an exemplaryembodiment, medial surface area SAM_(X)′ may be as little as 986 mm² ormay be as much as 2404 mm², or may be any area within the range definedby the foregoing values.

Tables 3 and 4 below illustrate that medial surface area SAM_(X)occupies a greater percentage of the total surface area contained withinperiphery 200 _(X), regardless of whether PCL cutout 28 is included inthe calculation. That is to say, medial fill area 82 is larger thanlateral fill area 80 by approximately the same proportion as medial andlateral surfaces areas SAM_(X), SAL_(X). In the exemplary embodiments ofFIG. 3A, medial surface area SAM_(X) occupies between 52% and 53% of thetotal surface area regardless, while lateral surface area SAM_(X)occupies the remainder. If the PCL cutout is excluded from thecalculation as shown in FIG. 3D, medial surface area SAM_(X)′ occupiesbetween 52% and 54% of the total surface area, while lateral surfacearea SAM_(X)′ occupies the remainder. With or without the PCL cutoutincluded in the calculation, it is contemplated that medial surfaceareas SAM_(X), SAM_(X)′ may occupy as little as 51% of the total surfacearea, and as much as 60% of the total surface area.

TABLE 3 Medial vs. Lateral Tibial Baseplate Surface Areas for Baseplateswith a PCL Cutout (FIGS. 2A and 3A) With PCL Notch Medial Surface AreaSAM_(x) Size as % of Total Surface Area 1/A 52% 2/B 52% 3/C 52% 4/D 52%5/E 52% 6/F 52% 7/G 53% 8/H 53% 9/J 53%

TABLE 4 Medial vs. Lateral Tibial Baseplate Surface Areas for Baseplateswithout a PCL Cutout (FIG. 3D) Without PCL Notch Medial Surface AreaSAM_(x)' Size as % of Total Surface Area 1/A 53% 2/B 52% 3/C 53% 4/D 53%5/E 53% 6/F 53% 7/G 53% 8/H 54% 9/J 54%

c. Anteroposterior Extent of Medial and Lateral Compartments

Still another way to characterize and quantify the asymmetry of tibialperiphery 200 is to compare the overall anteroposterior extent oflateral and medial compartments 20, 22.

Turning to FIG. 2A (which is drawn to scale, according to scales 230 and232) and FIG. 2B, lateral compartment 20 of tibial plateau 18 definesoverall lateral anteroposterior extent DAPL_(X), while medialcompartment 22 of tibial plateau 18 defines overall medialanteroposterior extent DAPM_(X), where X is an integer between 1 and 9corresponding to a particular component size as shown in FIG. 2A, asnoted above. As illustrated in Table 5 below, lateral anteroposteriorextent DAPL_(X) is less than medial anteroposterior extent DAPM_(X), forall component sizes.

This disparity in anteroposterior extent can be said to result frommedial compartment 22 extending posteriorly further than lateralcompartment 20. In the illustrative embodiment of FIG. 2B, lateralanteroposterior extent DAPL_(X) extends from anterior edge 202 tolateral posterior edge 204, while medial anteroposterior extent DAPM_(X)extends from anterior edge 202 to medial posterior edge 206. Thus, ifone takes anterior edge 202 to be the anteroposterior “zero point,” theadditional anteroposterior extent defined by medial compartment 22 isdue entirely to the further posterior position of medial posterior edge206.

As set forth in the right-hand column of Table 5, exemplary embodimentsof tibial baseplate 12 may define medial anteroposterior extent DAPM_(X)that is larger than lateral anteroposterior extent DAPL_(X) by as littleas 12.1%, 12.2% or 12.4%, and as much as 13.7%, 14.2% or 14.5%. It iscontemplated that such disparity between medial and lateralanteroposterior extents DAPM_(X), DAPL_(X) may be any percentage withinany range defined by the listed values of Table 5. Advantageously, theparticular asymmetric arrangement of tibial baseplate 12 with respect toanteroposterior extent of lateral and medial compartments 20, 22facilitates substantially complete coverage of tibia T, withoutoverhanging the edge of tibia T, in a wide variety of patients.

TABLE 5 Overall A/P and M/L Dimensions for Tibial Baseplates (FIGS. 2Aand 2B) Growth in A/P Medial Growth in A/P Lateral Additional A/PDimension (DAPM), Dimension (DAPL), Extent of DAPM Size fromnext-smaller size, from next-smaller size, vs. DAPL, % of (X) mm mm DAPL1/A — — 14.5% 2/B 2.3 2.13 14.2% 3/C 2.4 2.25 13.7% 4/D 2.3 2.27 13.1%5/E 3 2.8 12.7% 6/F 3.1 2.85 12.4% 7/G 3.2 2.81 12.5% 8/H 3.3 3.11 12.2%9/J 3.73 3.34 12.1%

For example, in an exemplary family of prosthesis sizes, at least 60%and as much as 90% coverage of the resected proximal surface is providedby tibial plateau 18 of tibial baseplate 12 when rotation is limited to+/−5 degrees from home axis A_(H). In a majority of all patients, suchcoverage is between 75-85%. Coverage of up to 100% may be achievedwithin the scope of the present disclosure, such as by fully extendingthe posterior-medial and anterior-lateral coverage of tibial plateau(which intentionally leave gaps between tibial plateau 18 and theperiphery of tibia T as noted herein).

The additional posteromedial material of tibial plateau 18 includeschamfer 32, described in detail below with respect to the assembly oftibial baseplate 12 to tibial bearing component 14. Chamfer 32 is formedin peripheral wall 25, such that chamfer 32 forms angle α (FIG. 8) withthe distal or bone-contacting surface 35 of tibial plateau 18. In theillustrated embodiment, chamfer 32 defines a substantially linearsagittal cross-sectional profile, with angle α between about 35 degreesand about 55 degrees. In addition, it is contemplated that chamfer 32may have an arcuate profile in a sagittal, coronal and/or transverseplane, and may include convex or concave curvature as required ordesired for a particular application.

2. Progressive Peripheral Growth Between Implant Sizes

In addition to the asymmetry of each individual size/embodiment oftibial baseplate 12, described in detail above, the present disclosurealso provides asymmetry in the way periphery 200 grows from one size tothe next. Advantageously, this asymmetric peripheral growth accommodatesobserved growth trends in tibias T of differently-sized patients, whilealso preserving the optimal fit and coverage provided by baseplate 12,and offering the other advantages of designs in accordance with thepresent disclosure as described herein.

In symmetrical peripheral growth, a larger size of baseplate is ascaled-up version of a smaller size and vice-versa. In the presentasymmetrical peripheral growth, by contrast, certain parameters oftibial baseplate 12 grow faster than others as the overall size of thebaseplate gets larger (i.e., from smallest size 1/A through largest size9/J). Thus, differently-sized components made in accordance with thepresent disclosure are not proportional to one another in all respects,in that a larger tibial prosthesis is not proportionally larger than asmaller tibial prosthesis in all aspects.

Referring now to FIG. 2B, periphery 200 _(X) defines centroid C_(X),which is medially biased with respect to home axis A_(H) owing to medialsurface area SAM being larger than lateral surface area SAL (asdescribed in detail above). Posterior-medial distance DMP_(X) extendsfrom centroid C_(X) toward the posterior-medial “corner” of periphery200 _(X) (i.e., toward posterior-medial corner arc 224, shown in FIG. 3Aand described above) at an angle of 130 counter-clockwise degrees fromhome axis A_(H). Similarly, posterior-lateral distance DLP_(X) extendsfrom centroid C_(X) toward the posterior-lateral “corner” of periphery200 _(X) (i.e., toward posterior-lateral corner arc 214, shown in FIG.3A and described above) at an angle of 120 clockwise degrees from homeaxis A_(H). The posterior-lateral and posterior-medial corners aredefined in a similar fashion as the anterior-lateral and anterior-medialcorners, described in detail above. Moreover, while the asymmetricposterior-medial and posterior lateral growth among consecutive sizes isdescribed below with respect to distances DLP_(X), DMP_(X), such growthoccurs in the entire area occupied by the posterior-medial andposterior-lateral corners.

As illustrated in FIG. 2A and shown in Table 6 below, lateral- andmedial-posterior distances DLP_(X), DMP_(X) do not grow linearly assmallest size 1/A progresses among consecutive sizes to eventually reachlargest size 9/J. Rather, lateral- and medial-posterior distancesDLP_(X), DMP_(X) exhibit an increase in the magnitude of growth as thesizes progress consecutively from size 1/A to size 9/J. This non-linear,asymmetric growth is illustrated in the graphs of FIGS. 2C and 2D and inTable 6 below.

TABLE 6 Growth of the Posterior-Medial and Posterior-Lateral Corners ofBaseplate Periphery (FIGS. 2A and 2B) Growth in medial-posterior Growthin lateral-posterior distance DMP_(x) from distance (DLP_(x)) fromcentroid (C_(x)), compared centroid (C_(x)), compared to next-smallersize, to next-smaller size, Size (X) mm mm 1 — — 2 2.42 2.48 3 2.56 2.8 4 2.76 2.55 5 2.86 3.26 6 3.71 2.64 7 3.28 2.83 8 3.52 2.28 9 3.76 3.29

In FIG. 2C, the amount of growth in DMP_(X) is plotted against size no.X. As illustrated, the family of tibial baseplates 12 illustrated inFIG. 2A exhibit a steadily increasing growth in DMP_(X), with nearly 20%average increase in growth from one size to the next consecutive size(as represented by the slope of the linear trend line having equationy=0.1975x+2.0225).

In FIG. 2D, the amount of growth in DLP_(X) is plotted against size no.X, and illustrates a smaller, but still positive growth increase acrossbaseplate sizes. More specifically, the family of tibial baseplates 12illustrated in FIG. 2A exhibit a nearly 4% average increase in growthfrom one size to the next consecutive size (as represented by the slopeof the linear trend line having equation y=0.0392x+2.5508).

As used herein, a “family” of prostheses refers to a set or kit ofprostheses sharing common geometrical and/or performancecharacteristics. For example, the family of nine tibial baseplates whoseperipheries 200 _(X) are shown in FIG. 2A share a common asymmetry asdescribed herein, such that each tibial baseplate is adapted to providesubstantial tibial coverage, facilitate proper implant rotation andavoid impingement with various soft tissues of the knee. Typically, afamily of prostheses includes a plurality of differently-sizedcomponents, with consecutively larger/smaller components sized toaccommodate a variety of differently-sized bones. In the exemplaryembodiments of the present disclosure, a size “1” or “A” prosthesis isthe smallest prosthesis of the family, a size “9” or “J” prosthesis isthe largest prosthesis of the family, and each of the intermediate sizes“2” or “B” through “8” or “H” are consecutively larger sizes.

Advantageously, in the family or kit of prosthesis peripheries shown inFIG. 2A, each tibial baseplate 12 (FIG. 1A) having periphery 200 _(X)provides a close match to a particular subset of patient tibias T havinga unique size and shape. Particular features of periphery 200 _(X) havebeen designed with non-linear growth which is calculated to provide theclosest possible fit for the largest number of particular naturalgeometries found in anatomic tibias T, as described in detail herein.This close fit allows for maximum coverage of the resected proximaltibial periphery 200 _(X), by accommodating the non-linear changes whichmay occur across anatomic tibial periphery sizes. Lateral- andmedial-posterior distances DLP_(X), DMP_(X) are exemplary non-lineargrowth parameters found in a family of tibial baseplates 12, and arereflective of non-linear growth in mediolateral extent DML_(X) andanteroposterior extents DAPM_(X) and DAPL_(X) across the various sizes.

3. PCL Cutout Aligned with Home Axis and Associated Technique

In the illustrated embodiment, tibial plateau 18 includes PCL cutout 28disposed between compartments 20, 22, as described above. PCL cutoutleaves PCL attachment point C_(P) accessible, thereby allowing the PCLto pass therethrough during and after implantation of tibial baseplate12. Tibial bearing component 14 (FIG. 5) may similarly include cutout30.

Thus, the illustrated embodiment of tibial prosthesis 10 is adapted fora cruciate retaining (CR) surgical procedure, in which the posteriorcruciate ligament is not resected during implantation of tibialprosthesis 10. Further, as noted above, home axis A_(H) includesreference to PCL attachment point C_(p) when tibial baseplate 12 ismounted upon tibia T. In order to facilitate alignment of home axisA_(H) with respect to tibial baseplate 12 and tibia T, alignment indicia70A, 70P (FIGS. 4A and 4B) may be marked on proximal surface 34 and/orperipheral wall 25. When tibial baseplate 12 is implanted (as describedbelow), anterior alignment indicia 70A (FIGS. 4A and 4B) is aligned withanterior point C_(A) at the “medial third” of the anterior tibialtubercle T, and posterior alignment indicia 70P is aligned with thenatural PCL attachment point C_(p) of tibia T.

However, it is contemplated that a prosthesis in accordance with thepresent disclosure may be made for a design in which the posteriorcruciate ligament is resected during surgery, such as “posteriorstabilized” (PS) or “ultra congruent” (UC) designs. The PS and UCdesigns may exclude PCL cutout 30 in bearing component 14, therebyobviating the need for PCL cutout 28 in tibial baseplate 12. Continuousmaterial may instead occupy cutout 28 (as schematically shown in FIG.3D). Moreover, it is contemplated that PCL cutouts 28, 30 may have anyshape and/or size within the scope of the present disclosure. Forexample, PCL cutouts 28, 30 may be asymmetrical with respect to ananteroposterior axis. For purposes of the present disclosure “bisecting”an asymmetric PCL cutout with an anteroposterior axis refers to dividingsuch cutout into two equal areas for a given anteroposterior section ofthe anteroposterior axis

4. Tibial Bearing Component and Deep Flexion Enablement

Turning again to FIG. 1A, tibial bearing component 14 includes lateralportion 39, medial portion 41, inferior surface 36 adapted to couple totibial baseplate 12, and superior surface 38 adapted to articulate withcondyles of a femoral component (such as femoral component 60 shown inFIG. 8 and described in detail below). Superior surface 38 includeslateral articular surface 40 in lateral portion 39 and medial articularsurface 42 in medial portion 41, with eminence 44 (FIG. 5) disposedbetween articular surfaces 40. 42. Referring to FIG. 5, eminence 44generally corresponds in shape and size with a natural tibial eminenceof tibia T prior to resection.

Referring now to FIG. 1A, tibial plateau 18 of tibial baseplate 12further includes a distal or bone contacting surface 35 and an opposingproximal or superior surface 34, with superior surface 34 having raisedperimeter 24 and locking mechanism 26 formed between lateral and medialcompartments 20, 22. Raised perimeter 24 and locking mechanism 26cooperate to retain tibial bearing component 14 upon tibial baseplate12, as described in detail below. Exemplary baseplate locking mechanismsare described in U.S. provisional patent application Ser. Nos.61/367,374 and 61/367,375, both entitled TIBIAL PROSTHESIS incorporatedby reference above in paragraph [0001].

Inferior surface 36 of tibial bearing component 14 includes recess 46 atthe periphery thereof and a tibial bearing locking mechanism (not shown)disposed between lateral and medial articular surfaces 40, 42. Exemplarybearing component locking mechanisms are disclosed in U.S. provisionalpatent application Ser. Nos. 61/367,374 and 61/367,375, both entitledTIBIAL PROSTHESIS. Recess 46 is sized and positioned to correspond withraised perimeter 24 of tibial plateau 18, and the tibial bearing lockingmechanism cooperates with locking mechanism 26 of tibial plateau 18 tofix tibial bearing component 14 to tibial baseplate 12 in a desiredposition and orientation as described in detail below. However, it iscontemplated that tibial bearing component 14 may be affixed tobaseplate 12 by any suitable mechanism or method within the scope of thepresent disclosure, such as by adhesive, dovetail tongue/groovearrangements, snap-action mechanisms, and the like.

As best seen in FIGS. 1B, 5 and 8, the outer periphery of tibial bearingcomponent 14 generally corresponds with the outer periphery of tibialplateau 18, except for the posteromedial extent of plateau 18 ascompared with tibial bearing component 14. The anterolateral “corner” oftibial bearing component 14 defines radius R₃ (FIG. 5) having agenerally common center with radius R2L of baseplate 12 in a transverseplane, i.e., radii R2L and R₃ are substantially coincident in a planview. Similarly, the anteromedial “corner” of tibial bearing component14 defines radius R₄ having a generally common center with radius R1R ofbaseplate 12 in a transverse plane, i.e., radii R1R and R₄ aresubstantially coincident in a plan view.

R₃ defines a slightly smaller radial length as compared to R2L, and R₄defines a slightly smaller radial length as compared to R1R, such thatthe anterior portion of perimeter wall 54 of tibial bearing component 14is set back from the anterior portion of peripheral wall 25 (i.e., fromanterior edge 202 and adjacent arcs, as described above) of tibialbaseplate 12. As with the above-described comparison between radii R2Land R1R, anteromedial radius R₄ is substantially larger thananterolateral radius R₃.

Given that medial portion 41 of tibial bearing component 14 has a lesseranteroposterior extent compared to medial compartment 22 of tibialplateau 18, medial portion 41 must be biased anteriorly in order for theanterior-medial “corners” of tibial bearing component 14 and tibialplateau 18 to coincide as shown in FIG. 5. In view of this anteriorbias, it may be said that tibial bearing component 14 is asymmetricallyoriented upon tibial plateau 18. More particularly, although lateralarticular surface 40 is generally centered with respect to lateralcompartment 20 of tibial plateau 18, medial articular surface 42 isanteriorly biased with respect to medial compartment 22 of tibialplateau 18 in order to leave chamfer 32 exposed at the posterior-lateralcorner. This asymmetric mounting of tibial bearing component 14 upontibial plateau 18 ensures a desired articular interaction between tibialprosthesis 10 and femoral component 60, as described in detail below.

Tibial plateau 18 of tibial baseplate 12 deviates from the periphery oftibial bearing component 14 in the posteromedial portion of eachcomponent, leaving medial portion 41 incongruent with medial compartment22 of tibial baseplate 12. More particularly, tibial plateau 18 extendsposteromedially to substantially cover the proximal resected surface oftibia T, as shown in FIG. 5 and described in above, while tibial bearingcomponent 14 does not extend posteromedially beyond the superiorterminus of chamfer 32 (i.e., tibial bearing component 14 does not“overhang” chamfer 32). In addition, tibial bearing component 14includes chamfer 50 formed in peripheral wall 54, with chamfer 50 havinga profile and geometrical arrangement corresponding with chamfer 32 oftibial plateau 18. More particularly, when tibial bearing component 14is assembled to tibial baseplate 12 as shown in FIGS. 1B and 8, theanterior orientation or “bias” of the medial portion of tibial bearingcomponent 14 (as described above) aligns chamfers 32, 50, which in turncooperate to create a substantially continuous chamfer extending fromtibia T to medial articular surface 42. Referring to FIG. 8, chamfers32, 50 further cooperate to define void 52 formed between femur F andtibial plateau 18 when tibial prosthesis 10 is in a deep flexionorientation. In the illustrated embodiment of FIG. 8, the deep flexionorientation is defined by angle β between anatomic tibia axis A_(T) andanatomic femoral axis A_(F) of up to about 25 degrees to about 40degrees, for example (i.e., about 140 degrees to 155 degrees of flexionor more).

Advantageously, void 52 cooperates with the “pulled back” or incongruentposterior medial edge 206 and posterior medial corner 224, as comparedto a typical tibial periphery (described above), to allow the deepflexion orientation to be achieved without impingement of femoralcomponent 60 and/or femur F upon tibial plateau 18 and/or tibial bearingcomponent 14. Soft tissues in the region of void 52 are therefore alsoaccommodated with little or no impingement on the surroundingcomponents.

In addition, the relatively large size of tibial plateau 18 (covering alarge proportion of the resected proximal surface of tibia T) alsoallows tibial bearing component 14 to be relatively large, so thattibial bearing component 14 provides sufficient non-articular surfacearea at chamfers 32, 50 and around the periphery of lateral and medialarticular surfaces 40, 42 to allow relatively large-radius, roundedtransitions between articular surfaces 40, 42 and peripheral wall 54 oftibial bearing component 14. These gradual, large-radius transitionsprevent undue friction between tibial prosthesis 10 and any surroundingsoft tissues which may remain in place after implantation of theprosthesis, such as the iliotibial (IT) band.

In certain ranges of prosthesis articulation, for example, the humaniliotibial (IT) band may touch the anterolateral “corner”, i.e., theportion of tibial bearing component 14 having radius R₃. Because theanterolateral extent of tibial bearing component 14 follows theanterolateral extent of tibial plateau 18 (as described above), thetransition between lateral articular surface 40 and peripheral wall 54at the point of contact between an IT band and tibial bearing component14 can have a relatively large convex portion while still leavingsufficient concave space for articular surface 40. This large convexportion results in a large contact area if the IT band does contacttibial bearing component 14, which in turn results in relatively lowpressures on the IT band. Further, the anterolateral “pull back” orincongruence between the anterior-lateral corner arc 210 of periphery200 and a typical tibial periphery, described in detail above, allowsthe corresponding anterior-lateral corner of bearing component 14 tomaintain separation from the IT band through a wide range of flexion,and low contact pressures where contact does occur.

However, to any such contact between the IT band and tibial bearingcomponent 14 may be avoided or minimized by designing periphery 200 suchthat anterior-lateral corner arc 210 and/or lateral edge arc 212 isbrought away from the expected periphery of a typical tibia T (ascalculated from anatomical data, described above). This extra spacingdesigned into periphery 200 provides extra clearance for the iliotibialband. In addition, this extra clearance assures that the substantialproportion of prospective patients lacking Gerdy's tubercle, which is aneminence located at the anterior-lateral portion of tibia T, will notexperience any “overhang” of tibial plateau 18 beyond the anatomicperiphery of resected tibia T.

Thus, generally speaking, tibial prosthesis 10 can be considered “softtissue friendly” because the edges of tibial bearing component 14 andtibial plateau 18, including chamfers 32, 50, are smooth and rounded, sothat any soft tissue coming into contact with these edges will be lesslikely to chafe or abrade.

Advantageously, the relatively large inferior/distal surface area oftibial plateau 18 facilitates a large amount of bone ingrowth where boneingrowth material is provided in tibial baseplate 12. For example,baseplate 12 may also be constructed of, or may be coated with, a highlyporous biomaterial. A highly porous biomaterial is useful as a bonesubstitute and as cell and tissue receptive material. A highly porousbiomaterial may have a porosity as low as 55%, 65%, or 75% or as high as80%, 85%, or 90%. An example of such a material is produced usingTrabecular Metal™ Technology generally available from Zimmer, Inc., ofWarsaw, Ind. Trabecular Metal™ is a trademark of Zimmer, Inc. Such amaterial may be formed from a reticulated vitreous carbon foam substratewhich is infiltrated and coated with a biocompatible metal, such astantalum, by a chemical vapor deposition (“CVD”) process in the mannerdisclosed in detail in U.S. Pat. No. 5,282,861 to Kaplan, the entiredisclosure of which is expressly incorporated herein by reference. Inaddition to tantalum, other metals such as niobium, or alloys oftantalum and niobium with one another or with other metals may also beused.

Generally, the porous tantalum structure includes a large plurality ofligaments defining open spaces therebetween, with each ligamentgenerally including a carbon core covered by a thin film of metal suchas tantalum, for example. The open spaces between the ligaments form amatrix of continuous channels having no dead ends, such that growth ofcancellous bone through the porous tantalum structure is uninhibited.The porous tantalum may include up to 75%, 85%, or more void spacetherein. Thus, porous tantalum is a lightweight, strong porous structurewhich is substantially uniform and consistent in composition, andclosely resembles the structure of natural cancellous bone, therebyproviding a matrix into which cancellous bone may grow to providefixation of implant [#] to the patient's bone.

The porous tantalum structure may be made in a variety of densities inorder to selectively tailor the structure for particular applications.In particular, as discussed in the above-incorporated U.S. Pat. No.5,282,861, the porous tantalum may be fabricated to virtually anydesired porosity and pore size, and can thus be matched with thesurrounding natural bone in order to provide an improved matrix for boneingrowth and mineralization.

5. Trial Tibial Components

Tibial prosthesis 10 may be provided in a variety of sizes andconfigurations to accommodate different bone sizes and geometries. Thechoice of one particular size may be planned preoperatively such asthrough preoperative imaging and other planning procedures.Alternatively, an implant size may be chosen, or a previous size choicemodified, intraoperatively. To facilitate proper intraoperativeselection of a particular size for tibial prosthesis 10 from among thefamily of sizes shown in FIG. 2A, and to promote proper orientation ofthe chosen prosthesis 10, tibial prosthesis 10 may be part of a kitincluding one or more template or “sizing” components.

Referring now to FIGS. 6 and 7, trial prosthesis 100 may be temporarilycoupled to tibia T for intraoperative sizing evaluation of tibialprosthesis 10 and initial steps in the implantation of tibial prosthesis10. Trial prosthesis 100 is one of a set of trial prostheses provided asa kit, with each trial prosthesis having a different size andgeometrical configuration. Each trial prosthesis in the set of trialprostheses corresponds to a permanent prosthesis 10, such as sizes1/A-9/J of tibial baseplate 12 as described above.

For example, as shown in FIG. 6, trial prosthesis 100 defines superiorsurface 112 generally corresponding in size and shape to proximalsurface 34 of tibial plateau 18, and including lateral portion 102 andmedial portion 104. Superior surface 112 is asymmetrical about home axisA_(H), with lateral portion 102 having a generally shorter overallanteroposterior extent as compared to medial portion 104 (which includesvoid indicator 106, discussed below). In addition, the anterolateral“corner” of lateral portion 102 defines radius R2L, which is identicalto radius R2L of tibial plateau 18, while the anteromedial “corner” ofmedial portion 104 defines radius R1R, which is identical to radius R1Rof tibial plateau 18 and greater than radius R2L.

Moreover, perimeter wall 114 of trial prosthesis 100 is substantiallyidentical to peripheral wall 25 of tibial plateau 18, and thereforedefines periphery 200 with the same features and shapes of perimeter 200described above with respect to tibial plateau 18. Thus, trialprosthesis 100 is asymmetrical about home axis A_(H) in a similar mannerto tibial plateau 18 of tibial baseplate 12, with the nature of thisasymmetry changing across the various other sizes of tibial prosthesisprovided in the kit including trial prosthesis 100.

In an alternative embodiment, a trial prosthesis may be provided whichextends completely to the posterior-medial edge of the natural tibialresection periphery. Thus, such a trial would substantially completelycover the resected tibial surface, thereby aiding in determination of aproper rotational orientation of the trial (and, therefore, of the finaltibial baseplate 12). In this alternative embodiment, the trialprosthesis lacks the posterior-medial “pull back” of tibial plateau 18,described above.

Trial prosthesis 100 includes void indicator 106 disposed at theposterior portion of medial portion 104, consuming a given posteromedialarea of superior surface 34 and peripheral wall 25. Void indicator 106indicates where void 52 (discussed above) will be located with respectto tibia T after implantation of tibial prosthesis 10. Void indicator106 facilitates proper rotational and spatial orientation of trialprosthesis 100 on the resected proximal surface of tibia T by allowing asurgeon to visually match tibial bearing component 14 with trialprosthesis 100, as described in detail below. In the illustratedembodiment, void indicator 106 is an area of visual and/or tactilecontrast with the remainder of tibial plateau 18. This contrast mayinclude, for example, a contrasting color, texture, surface finish, orthe like, or may be formed by a geometric discrepancy such as a step orlip, for example.

Referring specifically to FIG. 6, trial prosthesis 100 further includesa plurality of peg hole locators 108 corresponding to the properlocation for peg holes in tibia T to receive pegs (not shown) extendinginferiorly from tibial plateau 18 of tibial baseplate 12.Advantageously, peg hole locators 108 allow a surgeon to demarcate theproper center for peg holes in tibia T once the proper size andorientation for trial prosthesis 100 has been found, as discussed indetail below. Alternatively, peg hole locators 108 may be used as drillguides to drill appropriately positioned peg holes while trialprosthesis is still positioned on tibia T.

6. Tibial Prosthesis Implantation

In use, a surgeon first performs a resection of tibia T usingconventional procedures and tools, as are well-known in the art. In anexemplary embodiment, a surgeon will resect the proximal tibia to leavea planar surface prepared for receipt of a tibial baseplate. This planarsurface may define a tibial slope, which is chosen by the surgeon. Forexample, the surgeon may wish to perform a resection resulting inpositive tibial slope in which the resected tibial surface slopesproximally from posterior to anterior (i.e., the resected surface runs“uphill” from posterior to anterior). Alternatively, the surgeon mayinstead opt for negative tibial slope in which the resected tibialsurface slopes distally from posterior to anterior (i.e., the resectedsurface runs “downhill” from posterior to anterior). Varus or valgusslopes may also be employed, in which the resected surface slopesproximally or distally from medial to lateral. The choice of a tibialand/or varus/valgus slope, and the amount or angle of such slopes, maydepend upon a variety of factors including correction of deformities,mimicry of the native/preoperative tibial slope, and the like.

In an exemplary embodiment, keel 16 (FIG. 4B) defines a 5-degree,anteriorly-extending angle with respect to bone-contacting surface 35 oftibial plateau 18. Tibial baseplate 12 is appropriate for use with apositive tibial slope of as little as zero degrees and as much as 9degrees, and with a varus or valgus slope of up to 3 degrees. However,it is contemplated that a tibial baseplate made in accordance with thepresent disclosure may be used with any combination of tibial and/orvarus/valgus slopes, such as by changing the angular configuration ofthe keel with respect to the bone-contacting surface.

With a properly resected proximal tibial surface, the surgeon selectstrial prosthesis 100 from a kit of trial prostheses, with eachprosthesis in the kit having a different size and geometricalconfiguration (as discussed above). Trial prosthesis 100 is overlaid onthe resected surface of tibia T. If trial prosthesis 100 isappropriately sized, a small buffer zone 110 of exposed bone of resectedtibia T will be visible around the periphery of trial prosthesis 100.Buffer 110 is large enough to allow a surgeon to rotate and/orreposition trial prosthesis 100 within a small range, thereby offeringthe surgeon some flexibility in the final positioning and kinematicprofile of tibial prosthesis 10. However, buffer 110 is small enough toprevent trial prosthesis 100 from being rotated or moved to an improperlocation or orientation, or from being implanted in such as way as toproduce excessive overhang of the edge of trial prosthesis 100 past theperiphery of the resected tibial surface. In one exemplary embodiment,for example, trial prosthesis may be rotated from a centered orientationby up to +/−5 degrees (i.e., in either direction), though it iscontemplated that such rotation may be as much as +/−10 degrees or +/−15degrees.

To aid in rotational orientation, trial prosthesis may include anteriorand posterior alignment indicia 70A, 70P, which are the same marks inthe same location as indicia 70A, 70P provided on tibial plateau 18 asdescribed above. The surgeon can align indicia 70A with anterior pointC_(A) and indicia 70P with PCL attachment point C_(P), in similarfashion as described above, to ensure the anatomical and component homeaxes A_(H) are properly aligned. Alternatively, a surgeon may useindicia 70A, 70P to indicate a desired deviance from alignment with homeaxis A_(H). As noted above, deviation of up to 5 degrees is envisionedwith the exemplary embodiments described herein. A surgeon may choose toorient indicia 70A, 70P to another tibial landmark, such as the middleof the patella or the medial end of tibial tubercle B.

Thus, the large coverage of trial prosthesis 100 (and, concomitantly, oftibial plateau 18) ensures that tibial baseplate 12 will be properlypositioned and oriented on tibia T upon implantation, thereby ensuringproper kinematic interaction between tibial prosthesis 10 and femoralcomponent 60. If buffer zone 110 is either nonexistent or too large,another trial prosthesis 100 is selected from the kit and compared in asimilar fashion. This process is repeated iteratively until the surgeonhas a proper fit, such as the fit illustrated in FIGS. 6 and 7 betweentrial prosthesis 100 and tibia T.

With the proper size for trial prosthesis 100 selected and itsorientation on tibia T settled, trial prosthesis 100 is secured to tibiaT, such as by pins, screws, temporary adhesive, or any otherconventional attachment methods. Once trial prosthesis is so secured,other trial components, such as trial femoral components and trialtibial bearing components (not shown) may be positioned and used toarticulate the leg through a range of motion to ensure a desiredkinematic profile. During such articulation, void indicator 106indicates to the surgeon that any impingement of femoral component 60and/or femur F upon trial prosthesis 100 at void indicator 106 will notoccur when tibial prosthesis 10 is implanted. Once the surgeon issatisfied with the location, orientation and kinematic profile of trialprosthesis 100, peg hole locators 108 may be used to demarcate theappropriate location of peg holes in tibia T for tibial baseplate 12.Such peg holes may be drilled in tibia T with trial prosthesis 100attached, or trial prosthesis 100 may be removed prior to drilling theholes.

With tibia T prepared for receipt of tibial prosthesis 10, tibialbaseplate 12 may be provided by the surgeon (such as from a kit orsurgical inventory), and is implanted on tibia T, with pegs fitting intoholes previously identified and demarcated using peg hole locators 108of trial prosthesis 100. Tibial baseplate 12 is selected from the familyof tibial baseplates illustrated in FIG. 2A to correspond with the trialcomponent 100 chosen, which ensures that tibial plateau 18 will cover alarge proportion of the resected proximal surface of tibia T, as trialprosthesis 100 did prior to removal. Tibial baseplate is affixed totibia T by any suitable method, such as by keel 16 (FIG. 4B), adhesive,bone-ingrowth material, and the like.

With tibial baseplate 12 installed, tibial bearing component 14 may becoupled with tibial baseplate 12 to complete tibial prosthesis 10.However, once attached, tibial bearing component 14 does not fully covertibial plateau 18 of tibial baseplate 12. Rather, tibial bearingcomponent 14 leaves a posteromedial portion of tibial baseplate 12uncovered to create void 52 (as shown in FIG. 8 and discussed above).Thus, a surgeon may wish to verify that this anterior-biased,“asymmetrical” orientation of medial articular surface 42 is properprior to permanent affixation of tibial bearing component 14 to tibialbaseplate 12.

To accomplish such verification, tibial bearing component 14 is placedside-by-side with trial prosthesis 100, with inferior surface 36 oftibial bearing component 14 in contact with superior surface 112 oftrial prosthesis 100. Tibial bearing component 14 will substantiallycover superior surface 112, but will not cover void indicator 106. Putanother way, peripheral wall 54 of tibial bearing component 14 willtrace perimeter wall 114 of tibial trial prosthesis 100, excluding theposteromedial area defined by void indicator 106. If inferior surface 36of tibial bearing component 14 is a match with superior surface 112 oftrial prosthesis 100 except for void indicator 106 (which is leftuncovered by tibial bearing component 14), then tibial bearing component14 is the proper size component and may be confidently installed upontibial plateau 18 of tibial baseplate 12.

Tibial baseplate 12 may then be implanted upon the proximal surface oftibia T in accordance with accepted surgical procedures. Exemplarysurgical procedures and associated surgical instruments are disclosed in“Zimmer LPS-Flex Fixed Bearing Knee, Surgical Technique,” “NEXGENCOMPLETE KNEE SOLUTION, Surgical Technique for the CR-Flex Fixed BearingKnee” and “Zimmer NexGen Complete Knee SolutionExtramedullary/Intramedullary Tibial Resector, Surgical Technique”(collectively, the “Zimmer Surgical Techniques”), copies of which aresubmitted on even date herewith, the entire disclosures of which arehereby expressly incorporated by reference herein.

When the surgeon is satisfied that tibial bearing component 14 isproperly matched and fitted to the installed tibial baseplate 12,bearing component 14 is secured using locking mechanism 26 and thecorresponding tibial bearing locking mechanism an appropriateinstrumentation (not shown). Proper location and rotational orientationof tibial bearing component 14 upon tibial plateau 18 is ensured byraised perimeter 24 cooperating with recess 46, and locking mechanism 26cooperating with the corresponding tibial bearing locking mechanism (notshown). Such proper orientation results in medial articular surface 42being generally anteriorly disposed with respect to medial compartment22 of tibial plateau 18.

Femoral component 60 may be affixed to a distal end of femur F, ifappropriate, using any conventional methods and/or components. Exemplarysurgical procedures and instruments for such affixation are disclosed inthe Zimmer Surgical Techniques, incorporated by reference above. Femur Fand tibia T may then be articulated with respect to one another toensure that neither femur F nor femoral component 60 impinges upontibial baseplate 12 and/or tibial bearing component 14 in deep flexion,such as at a flexion angle β of 155° as shown in FIG. 8. When thesurgeon is satisfied with the location, orientation and kinematicprofile of tibial prosthesis 10, the knee replacement surgery iscompleted in accordance with conventional procedures.

While this invention has been described as having an exemplary design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

1. A tibial prosthesis comprising: a distal surface; a proximal surfacegenerally opposite said distal surface, said proximal surface having alateral compartment and a medial compartment; and a peripheral wallextending between said distal and said proximal surface, said peripheralwall defining an anterior edge; a lateral posterior edge generallyopposite said anterior edge and forming a posterior boundary of saidlateral compartment; a medial posterior edge generally opposite saidanterior edge and forming a posterior boundary of said medialcompartment; a lateral periphery extending from said anterior edge tosaid lateral posterior edge, said lateral periphery defining a pluralityof adjacent lateral arcs, an adjacent pair of said plurality of adjacentlateral arcs defining a first lateral radius and a second lateralradius, respectively, said first lateral radius larger than said secondlateral radius by at least 100%, whereby said lateral periphery isrelatively boxy; and a medial periphery extending from said anterioredge to said medial posterior edge, said medial periphery defining aplurality of adjacent medial arcs, an adjacent pair of said plurality ofadjacent medial arcs defining a first medial radius and a second medialradius, respectively, said first medial radius larger than said secondmedial radius by less than 75%, whereby said medial periphery isgenerally rounded.
 2. The tibial prosthesis of claim 1, wherein saidlateral periphery comprises: a lateral edge defining a substantiallyperpendicular tangent with respect to said anterior edge, said lateraledge defining a lateral edge radius, an anterior-lateral cornertraversing an angular sweep between said anterior edge and said lateraledge, said anterior-lateral corner defining an anterior-lateral cornerradius, said lateral edge radius larger than said anterior-lateralcorner radius by at least 42%.
 3. The tibial prosthesis of claim 2,wherein said lateral edge radius is larger than said anterior-lateralcorner radius by up to 142%.
 4. The tibial prosthesis of claim 1,wherein said lateral periphery comprises: a lateral edge defining asubstantially perpendicular tangent with respect to said anterior edge,said lateral edge defining a lateral edge radius, a posterior-lateralcorner traversing an angular sweep between said lateral posterior edgeand said lateral edge, said posterior-lateral corner defining aposterior-lateral corner radius, said lateral edge radius larger thansaid posterior-lateral corner radius by at least 198%.
 5. The tibialprosthesis of claim 4, wherein said lateral edge radius is larger thansaid posterior-lateral corner radius by up to 324%.
 6. The tibialprosthesis of claim 1, wherein said medial periphery comprises: a medialedge defining a substantially perpendicular tangent with respect to saidanterior edge, said medial edge defining a medial edge radius, ananterior-medial corner traversing an angular sweep between said anterioredge and said medial edge, said anterior-medial corner defining ananterior-medial corner radius, said medial edge radius larger than saidanterior-medial corner radius by up to 74%.
 7. The tibial prosthesis ofclaim 1, wherein said medial periphery comprises: a medial edge defininga substantially perpendicular tangent with respect to said anterioredge, said medial edge defining a medial edge radius, a posterior-medialcorner traversing an angular sweep between said medial posterior edgeand said medial edge, said posterior-medial corner defining aposterior-medial corner radius, said medial edge radius larger than saidposterior-medial corner radius by up to 61%.
 8. The tibial prosthesis ofclaim 1, wherein said plurality of adjacent lateral arcs each define arespective lateral radius value, each respective radius value differentfrom all of the other radius values.
 9. The tibial prosthesis of claim1, wherein at least one of said anterior edge, said lateral posterioredge and said medial posterior edge is planar.
 10. The tibial prosthesisof claim 1, wherein said lateral periphery comprises: a lateral edgedefining a substantially perpendicular tangent with respect to saidanterior edge; an anterior-lateral corner traversing an angular sweepbetween said anterior edge and said lateral edge; and an arcuateanterior section between said anterior edge and said anterior-lateralcorner, said arcuate anterior section defining one of said plurality ofadjacent lateral arcs.
 11. The tibial prosthesis of claim 1, whereinsaid plurality of adjacent lateral arcs are greater in number ascompared to said plurality of adjacent medial arcs.
 12. The tibialprosthesis of claim 1, wherein said plurality of adjacent lateral arcscomprises at least five adjacent lateral arcs.
 13. The tibial prosthesisof claim 1, wherein said plurality of adjacent medial arcs comprises upto three adjacent lateral arcs.
 14. The tibial prosthesis of claim 1,wherein: said lateral periphery comprises: a lateral edge defining asubstantially perpendicular tangent with respect to said anterior edge;and an anterior-lateral corner traversing a first angular sweep betweensaid anterior edge and said lateral edge, said anterior-lateral cornerdefining an anterior-lateral corner radius, said medial peripherycomprises: a medial edge defining a substantially perpendicular tangentwith respect to said anterior edge; and an anterior-medial cornertraversing a second angular sweep between said anterior edge and saidmedial edge, said anterior-medial corner defining an anterior-medialcorner radius, said first angular sweep similar to said second angularsweep, said anterior-medial corner radius larger than saidanterior-lateral corner radius.
 15. The tibial prosthesis of claim 14,wherein said anterior-medial corner radius is larger than saidanterior-lateral corner radius by at least 48%.
 16. The tibialprosthesis of claim 14, wherein said anterior-medial corner radius islarger than said anterior-lateral corner radius by up to 149%.
 17. Thetibial prosthesis of claim 1, wherein said tibial prosthesis comprises atibial baseplate.
 18. The tibial prosthesis of claim 1, wherein saidtibial prosthesis comprises a tibial baseplate and a tibial bearingcomponent, said tibial baseplate including an anterior-lateral cornerdefining an anterior-lateral corner radius having a first radial center,and an anterior-medial corner defining an anterior-medial corner radiushaving a second radial center, said tibial bearing component comprising:a lateral portion defining an anterior-lateral bearing corner defining athird radius having a third radial center, said third radial centersubstantially coincident with said first radial center in a transverseplane when said tibial bearing component is mounted to said tibialbaseplate, said third radius smaller than said anterior-lateral cornerradius; and a medial portion defining an anterior-medial bearing cornerdefining a fourth radius having a fourth radial center, said fourthradial center substantially coincident with said second radial center inthe transverse plane when said tibial bearing component is mounted tosaid tibial baseplate, said fourth radius smaller than saidanterior-medial corner radius, said fourth radius substantially largerthan said third radius.
 19. The tibial prosthesis of claim 1, wherein:said medial compartment defines a medial anteroposterior extentextending from said anterior edge of said peripheral wall to said medialposterior edge, and said lateral compartment defines a lateralanteroposterior extent extending from said anterior edge of saidperipheral wall to said lateral posterior edge, said medialanteroposterior extent larger than said lateral anteroposterior extent.20. The tibial prosthesis of claim 1, wherein: said lateral periphery isasymmetric with respect to said medial periphery about ananteroposterior axis.
 21. The tibial prosthesis of claim 20, whereinsaid anteroposterior axis is aligned with a home axis when said tibialprosthesis is mounted to a tibia, said home axis defined as a lineextending from a posterior point at a geometric center of an attachmentarea between a posterior cruciate ligament and the tibia, to an anteriorpoint disposed on an anterior tubercle of the tibia, the tubercle havinga tubercle width W, said anterior point disposed on the tubercle at alocation medially spaced from a midpoint of the tubercle by an amountequal to W/6.
 22. The tibial prosthesis of claim 20, wherein: saidtibial prosthesis includes a PCL cutout area generally opposite saidanterior edge and between said lateral periphery and said medialperiphery, and said anteroposterior axis bisects said anterior edge andbisects said PCL cutout area.
 23. The tibial prosthesis of claim 22,wherein: said medial periphery cooperates with said anteroposterior axisto bound a medial surface area, said lateral periphery cooperates withsaid anteroposterior axis to bound a lateral surface area, and saidmedial surface area is larger than said lateral surface area.
 24. Thetibial prosthesis of claim 20, wherein: said lateral periphery includesan anterior-lateral corner defining an anterior-lateral corner radiushaving a first radial center, said medial periphery includes ananterior-medial corner defining an anterior-medial corner radius havinga second radial center, and a mediolateral axis defining the longestline segment bounded by said peripheral wall that is also perpendicularto said anteroposterior axis, said first radial center disposed betweensaid mediolateral axis and said anterior edge, said second radial centerdisposed posterior of said mediolateral axis.
 25. The tibial prosthesisof claim 1, wherein said tibial prosthesis is sized and shaped to coverbetween about 60% and about 90% of a resected proximal surface of atibia to create a buffer zone on all sides between a perimeter of asurface of the tibia and the peripheral wall.
 26. A tibial prosthesiscomprising: a distal surface; a proximal surface generally opposite saiddistal surface; and a peripheral wall extending between said distal andsaid proximal surface, said peripheral wall defining an anterior edge; alateral periphery including: a lateral edge defining a substantiallyperpendicular tangent with respect to said anterior edge, ananterior-lateral corner traversing an angular sweep between saidanterior edge and said lateral edge to define a boxy corner peripheryhaving an anterior-lateral corner edge length, and a posterior-lateralcorner extending away from said lateral edge and said anterior-lateralcorner; and a medial periphery including: a medial edge defining asubstantially perpendicular tangent with respect to said anterior edge,an anterior-medial corner traversing an angular sweep between saidanterior edge and said medial edge to define a rounded corner peripheryhaving an anterior-medial corner edge length that is longer than theanterior-lateral corner edge length, in which the angular sweep betweensaid anterior edge and said medial edge is similar to the angular sweepbetween said anterior edge and said lateral edge, and a posterior-medialcorner extending away from said medial edge and said anterior-medialcorner.
 27. The tibial prosthesis of claim 26, wherein: said lateraledge comprises a lateral edge arc defining a lateral edge radius, saidanterior-lateral corner comprises an anterior-lateral corner arcdefining an anterior-lateral corner radius, and said lateral edge radiuslarger than said anterior-lateral corner radius by at least 42%.
 28. Thetibial prosthesis of claim 27, wherein said lateral edge radius islarger than said anterior-lateral corner radius by up to 142%.
 29. Thetibial prosthesis of claim 26, wherein: said lateral edge comprises alateral edge arc defining a lateral edge radius, said posterior-lateralcorner comprises a posterior-lateral corner arc defining aposterior-lateral corner radius, and said lateral edge radius largerthan said posterior-lateral corner radius by at least 198%.
 30. Thetibial prosthesis of claim 29, wherein said lateral edge radius islarger than said posterior-lateral corner radius by up to 324%.
 31. Thetibial prosthesis of claim 26, wherein: said medial edge comprises amedial edge arc defining a medial edge radius, said anterior-medialcorner comprises an anterior-medial corner arc defining ananterior-medial corner radius, and said medial edge radius larger thansaid anterior-medial corner radius by up to 74%.
 32. The tibialprosthesis of claim 26, wherein: said medial edge comprises a medialedge arc defining a medial edge radius, said posterior-medial cornercomprises a posterior-medial corner arc defining a posterior-medialcorner radius, and said medial edge radius larger than saidposterior-medial corner radius by up to 61%.
 33. The tibial prosthesisof claim 26, wherein: said anterior-medial corner defines ananterior-medial corner arc defining an anterior-medial corner radius,said anterior-lateral corner defines an anterior-lateral corner arcdefining an anterior-lateral corner radius, said anterior-medial cornerradius different from said anterior-lateral corner radius.
 34. Thetibial prosthesis of claim 26, wherein said lateral periphery defines anarcuate anterior section between said anterior edge and saidanterior-lateral corner.
 35. The tibial prosthesis of claim 26, wherein:said lateral periphery defines a plurality of adjacent lateral arcs,said medial periphery defines a plurality of adjacent medial arcs, saidplurality of adjacent lateral arcs greater in number as compared to saidplurality of adjacent medial arcs.
 36. The tibial prosthesis of claim35, wherein said plurality of adjacent lateral arcs comprises at leastfive adjacent lateral arcs.
 37. The tibial prosthesis of claim 35,wherein said plurality of adjacent medial arcs comprises up to threeadjacent lateral arcs.
 38. The tibial prosthesis of claim 26, wherein:said anterior-lateral corner comprises an anterior-lateral corner arcdefining an anterior-lateral corner radius, and said anterior-medialcorner comprises an anterior-medial corner arc defining ananterior-medial corner radius, said anterior-medial corner radius largerthan said anterior-lateral corner radius.
 39. The tibial prosthesis ofclaim 26, wherein said tibial prosthesis comprises a tibial baseplate,said distal surface comprising a bone contacting surface and saidproximal surface comprising a tibial bearing engagement surface.
 40. Thetibial prosthesis of claim 39, wherein said tibial baseplate is sizedand shaped to cover between about 60% and about 90% of a resectedproximal surface of a tibia to create a buffer zone on all sides betweena perimeter of a surface of the tibia and the peripheral wall.
 41. Thetibial prosthesis of claim 39, wherein: said anterior-lateral cornercomprises an anterior-lateral corner arc defining an anterior-lateralcorner radius having a first radius center, and said anterior-medialcorner comprises an anterior-medial corner arc defining ananterior-medial corner radius having a second radius center, said tibialbaseplate in combination with a tibial bearing component mountable tosaid tibial baseplate, said tibial bearing component comprising: alateral portion defining an anterior-lateral bearing corner defining athird radius having a third radius center, said third radius centersubstantially coincident with said first radius center in a transverseplane when said tibial bearing component is mounted to said tibialbaseplate, said third radius smaller than said anterior-lateral cornerradius; and a medial portion defining an anterior-medial bearing cornerdefining a fourth radius having a fourth radius center, said fourthradius center substantially coincident with said second radius center inthe transverse plane when said tibial bearing component is mounted tosaid tibial baseplate, said fourth radius smaller than saidanterior-medial corner radius, said fourth radius substantially largerthan said third radius.
 42. The tibial prosthesis of claim 26, wherein:said lateral periphery is asymmetric with respect to said medialperiphery about an anteroposterior axis.
 43. The tibial prosthesis ofclaim 42, wherein said anteroposterior axis is aligned with a home axiswhen said tibial prosthesis is mounted to a tibia, said home axisdefined as a line extending from a posterior point at a geometric centerof an attachment area between a posterior cruciate ligament and thetibia, to an anterior point disposed on an anterior tubercle of thetibia, the tubercle having a tubercle width W, said anterior pointdisposed on the tubercle at a location medially spaced from a midpointof the tubercle by an amount equal to W/6.
 44. The tibial prosthesis ofclaim 42, wherein: said tibial prosthesis includes a PCL cutout areagenerally opposite said anterior edge and between said lateral peripheryand said medial periphery, and said anteroposterior axis bisects saidanterior edge and bisects said PCL cutout area.
 45. The tibialprosthesis of claim 42, wherein: said medial periphery cooperates withsaid anteroposterior axis to bound a medial surface area, said lateralperiphery cooperates with said anteroposterior axis to bound a lateralsurface area, and said medial surface area is larger than said lateralsurface area.
 46. The tibial prosthesis of claim 42, wherein: saidanterior-lateral corner comprises an anterior-lateral corner arcdefining an anterior-lateral corner radius having a first radius center,and said anterior-medial corner comprises an anterior-medial corner arcdefining an anterior-medial corner radius having a second radius center,a mediolateral axis defining the longest line segment within saidperipheral wall that is also perpendicular to said anteroposterior axis,said first radius center disposed between said mediolateral axis andsaid anterior edge, said second radius center disposed posterior of saidmediolateral axis.
 47. The tibial prosthesis of claim 26, wherein: saidmedial periphery defines a medial posterior edge generally opposite saidanterior edge, a medial anteroposterior extent extending from saidanterior edge to said medial posterior edge, and said lateral peripherydefines a lateral posterior edge generally opposite said anterior edge,a lateral anteroposterior extent extending from said anterior edge tosaid lateral posterior edge, said medial anteroposterior extent largerthan said lateral anteroposterior extent.
 48. The tibial prosthesis ofclaim 26, wherein said anterior edge is substantially planar.
 49. Thetibial prosthesis of claim 26, wherein said tibial prosthesis comprisesa tibial bearing component, said distal surface comprising a tibialbaseplate engagement surface and said proximal surface comprising anarticular surface adapted to articulate with a femoral component.
 50. Atibial prosthesis comprising an asymmetric prosthesis periphery, saidperiphery comprising: an anteroposterior axis dividing said prosthesisperiphery into a medial compartment and a lateral compartment; ananterior edge disposed between said medial compartment and said lateralcompartment; a lateral posterior edge generally opposite said anterioredge and forming a posterior boundary of said lateral compartment; amedial posterior edge generally opposite said anterior edge and forminga posterior boundary of said medial compartment; a lateral peripheryextending from said anterior edge to said lateral posterior edge, saidlateral periphery defining: an anterior-lateral arc having ananterior-lateral arc center; and a lateral arc having a lateral arccenter, said lateral arc defining a tangent parallel to saidanteroposterior axis; a medial periphery extending from said anterioredge to said medial posterior edge, said medial periphery defining: ananterior-medial arc having an anterior-medial arc center; and a medialarc having a medial arc center, said medial arc defining a tangentparallel to said anteroposterior axis, a mediolateral axis defining thelongest line segment within said prosthesis periphery that is alsoperpendicular to said anteroposterior axis, said anterior-lateral arccenter disposed between said mediolateral axis and said anterior edge,said anterior-medial arc center disposed posterior of said mediolateralaxis.
 51. The tibial prosthesis of claim 50, wherein: said lateral arcdefines a lateral arc radius, said anterior-lateral arc defines ananterior-lateral arc radius, said lateral arc radius larger than saidanterior-lateral arc radius by at least 42%.
 52. The tibial prosthesisof claim 51, wherein said lateral arc radius is larger than saidanterior-lateral arc radius by up to 142%.
 53. The tibial prosthesis ofclaim 50, wherein said lateral arc defines a lateral arc radius, saidlateral periphery comprising: a posterior-lateral arc traversing anangular sweep between said lateral posterior edge and said lateral arc,said posterior-lateral arc defining a posterior-lateral arc radius, saidlateral arc radius larger than said posterior-lateral arc radius by atleast 198%.
 54. The tibial prosthesis of claim 53, wherein said lateralarc radius is larger than said posterior-lateral arc radius by up to324%.
 55. The tibial prosthesis of claim 50, wherein: said medial arcdefines a medial arc radius, said anterior-medial arc defines ananterior-medial arc radius, said medial arc radius larger than saidanterior-medial arc radius by up to 74%.
 56. The tibial prosthesis ofclaim 50, wherein said medial arc defines a medial arc radius, saidmedial periphery comprising: a posterior-medial arc traversing anangular sweep between said medial posterior edge and said medial arc,said posterior-medial arc defining a posterior-medial arc radius, saidmedial arc radius larger than said posterior-medial arc radius by up to61%.
 57. The tibial prosthesis of claim 50, wherein saidanterior-lateral arc and said lateral arc each define respective lateralradii values that are different from one another.
 58. The tibialprosthesis of claim 50, wherein at least one of said anterior edge, saidlateral posterior edge and said medial posterior edge is planar.
 59. Thetibial prosthesis of claim 50, wherein said lateral periphery defines anarcuate anterior section between said anterior edge and saidanterior-lateral arc.
 60. The tibial prosthesis of claim 50, wherein:said lateral periphery defines a plurality of adjacent lateral arcsincluding said anterior-lateral arc and said lateral arc, and saidmedial periphery defines a plurality of adjacent medial arcs includingsaid anterior-medial arc and said medial arc, said plurality of adjacentlateral arcs greater in number as compared to said plurality of adjacentmedial arcs.
 61. The tibial prosthesis of claim 60, wherein saidplurality of adjacent lateral arcs comprises at least five adjacentlateral arcs.
 62. The tibial prosthesis of claim 60, wherein saidplurality of adjacent medial arcs comprises up to three adjacent lateralarcs.
 63. The tibial prosthesis of claim 50, wherein: saidanterior-lateral arc defines an anterior-lateral arc radius with a firstangular sweep, said anterior-medial arc defines an anterior-medial arcradius with a second angular sweep similar to said first angular sweep,said anterior-medial arc radius larger than said anterior-lateral arcradius.
 64. The tibial prosthesis of claim 50, wherein said tibialprosthesis comprises a tibial baseplate having a proximal surface and anopposing distal surface, said distal surface comprising a bonecontacting surface and said proximal surface comprising a tibial bearingengagement surface.
 65. The tibial prosthesis of claim 64, wherein saidtibial baseplate is sized and shaped to cover between about 60% andabout 90% of a resected proximal surface of a tibia to create a bufferzone on all sides between a perimeter of a surface of the tibia and saidprosthesis periphery.
 66. The tibial prosthesis of claim 50, incombination with a tibial bearing component mountable to said tibialprosthesis, said anterior-lateral arc defining an anterior-lateral arcradius, said anterior-medial arc defining an anterior-medial arc radius,said tibial bearing component comprising: a lateral portion defining ananterior-lateral bearing corner defining a third radius having a thirdradial center, said third radial center substantially coincident withsaid anterior-lateral arc center in a transverse plane when said tibialbearing component is mounted to said tibial prosthesis, said thirdradius smaller than said anterior-lateral arc radius; and a medialportion defining an anterior-medial bearing corner defining a fourthradius having a fourth radial center, said fourth radial centersubstantially coincident with said anterior-medial arc center in thetransverse plane when said tibial bearing component is mounted to saidtibial prosthesis, said fourth radius smaller than said anterior-medialarc radius, said fourth radius substantially larger than said thirdradius.
 67. The tibial prosthesis of claim 50, wherein: said peripherydefines a medial anteroposterior extent extending from said anterioredge to said medial posterior edge, and said periphery defines a lateralanteroposterior extent extending from said anterior edge to said lateralposterior edge, said medial anteroposterior extent larger than saidlateral anteroposterior extent.
 68. The tibial prosthesis of claim 50,wherein said anteroposterior axis is aligned with a home axis when saidtibial prosthesis is mounted to a tibia, said home axis defined as aline extending from a posterior point at a geometric center of anattachment area between a posterior cruciate ligament and the tibia, toan anterior point disposed on an anterior tubercle of the tibia, thetubercle having a tubercle width W, said anterior point disposed on thetubercle at a location medially spaced from a midpoint of the tubercleby an amount equal to W/6.
 69. The tibial prosthesis of claim 50,wherein: said tibial prosthesis includes a PCL cutout area generallyopposite said anterior edge and between said lateral periphery and saidmedial periphery, and said anteroposterior axis bisects said anterioredge and bisects said PCL cutout area.
 70. The tibial prosthesis ofclaim 50, wherein: said medial periphery cooperates with saidanteroposterior axis to bound a medial surface area, said lateralperiphery cooperates with said anteroposterior axis to bound a lateralsurface area, and said medial surface area is larger than said lateralsurface area.